1. Technical field
This invention relates to a Fourier transform lens system and a holographic storage system using the same, and more particularly to a holographic storage system where a signal beam and a reference beam co-propagate along a common optical axis.
2. Related Art
Among all of modern systems for data storage, holographic storage systems are believed by many to be the most promising due to their higher data storage densities, higher data transfer rates, and shorter access time.
In holographic storage systems, data is stored as holograms that result from the interference of a signal light beam and a reference light beam. Generally speaking, during recording, data can be encoded within the signal light beam by using an object generator, e.g., a spatial light modulator (SLM). Typically, an SLM is a two-dimensional matrix of pixels. Each pixel in the matrix can be directed to transmit or reflect light, which corresponds to the binary digit 1; or to block light, which corresponds to the binary digit 0. Once the signal light beam is encoded by the SLM, it passes through a Fourier transform lens system, and is incident on a holographic storage medium where it intersects with the reference beam to form an interference pattern (i.e., hologram). The interference pattern records the data encoded in the signal light beam to the holographic storage medium. During retrieval, the data recorded in the holographic storage medium is read by illuminating the storage medium with the reference beam. The reference beam diffracts off the stored hologram, generating a reconstructed signal light beam proportional to the original signal light beam used to store the hologram. The reconstructed signal light beam passes through a Fourier transform lens system, and is then typically imaged onto a sensor such as a CCD (charge coupled device) or a CMOS (complementary metal-oxide-semiconductor) active pixel array device. The sensor is attached to a decoder, which is capable of decoding the data contained in the reconstructed signal light beam.
Generally, the above-described recording process and retrieval process can be accomplished by a single integrated holographic storage system or by two separate holographic systems. In the case of two separate holographic systems, one of these is a holographic recording system for data recording, and the other is a holographic retrieval system for data retrieval. The implementation of holographic recording and retrieval techniques in a commercially viable storage system benefits from a simple and robust design of the Fourier transform lens systems. Additionally, various medium types and geometries, e.g., a holographic disk, a holographic tape, or bulk holographic material (e.g., a crystal), have to be supported by such a storage system.
One of proposed approaches for simplifying the optics of a holographic storage system involves combining the signal and reference beams and passing them substantially along a common optical axis through shared optical elements. In this approach, in general, a reference generator (e.g., a light diffuser) is located in a common plane with an object generator for producing a reference beam. The reference beam and signal beam co-propagate along the common optical axis through the shared optical elements, and are incident on a holographic storage medium to write a hologram therein. This configuration increases the clear apertures of the shared optical elements, such as the Fourier transform lens systems. Therefore the Fourier transform lens systems need to be configured appropriately.
What is needed is to provide a Fourier transform lens system and a holographic storage system using the same, wherein a signal beam and a reference beam co-propagate along a common optical axis.